"Ingemar Johansson S" <ingemar.s.johansson at ericsson.com> writes:
My question is ..how serious is the identified problem ?.
My gut feeling is that a receiver that implements e.g decoding of
G.718 content will likely update the RTP stack to recognize header
extensions if header extensions are used to carry e.g the NTP
timestamp related to the RTP timetstamp in the RTP header.
What if an intermediary deletes the header extension? (SBC,
B2BUA, raw message store, etc).
There is of course a risk that this may happen.
However I am curious as to what extent this may actually happen as it
would also cause the authentication to fail in case SRTP is used.
SRTP doesn't use header extensions. (I think ZRTP was doing so, but
I don't know if they kept that or if there were alternatives, and I
think ZRTP just would consider that a case where the channel couldn't be
secured if there's no alternative channel (RTCP?) for key exchange.)
There's no easy way to know (except maybe by asking the ZRTP people) how
often this happens. Certainly Asterisk and things like it would be a major
blocker to this (they act as kind-of B2BUA's -- don't get me started on
what a mess their implementations are). I haven't seen a true SBC that
stripped them - but on the other hand I haven't used header extensions
except occasionally.
This isn't to say it might not be a good idea - but requiring
header extensions to pass through may not be a good idea.
You also have issues where you can't use multiple header
extensions in the same RTP packet, so moving too much to them
is a problem.
Ok. A fallback is always to use RTCP SR (which is sent anyway), the
convergence will be slower and clock-skew compensation will complicate
an implementation somewhat but it will anyway be a fallback solution.
The additional method (if possible to use) will speed up convergenge.
As for the problem with multiple header extensions, I would not believe
that you need to add the "timing alignent" header extension in every
packet. Also I believe it is up to the sender to prioritize between the
header extensions.
That does cover the requirement that header extensions can't be mandatory,
though the details and consequences of the fallback need the be worked out.
Adding it to every packet is a big no-no as mentioned. Which packets would
it need to be added to, taking into account packet loss, startup, etc?
The scheduling issues may mean that these extensions don't get transmitted
for a while, but another issue is that the inserters of the extensions
aren't always coordinated -- witness ZRTP which in the current
implementation acts as a wrapper layer - if the main RTP stack adds an
extension, that means that ZRTP can't use that packet for an extension.
This should be ok, and ZRTP has to account for this possibility (as well as
packet loss).